One of the main tasks of the Transmission Control Protocol (TCP), which is the transport layer of the TCP/Internet protocol (IP) suite, is congestion control. For this purpose, TCP uses a number of mechanisms to achieve high performance and avoid situations of degrading network performance. These mechanisms control the rate of data entering the network, keeping the data flow below a rate that would trigger collapse. They also attempt to achieve an approximately fair allocation between different flows in the network.
TCP utilizes acknowledgments for successfully received data packets, or lack of such acknowledgments, for inferring network conditions along a data path between the sender and the receiver of a TCP connection. Based on transmission time, or round-trip time, measurements and utilizing timers, TCP senders and receivers can alter the flow of data packets through the network.
Low Extra Delay Background Transport (LEDBAT) is a delay-based congestion control mechanism which attempts to utilize the available bandwidth on an end-to-end path through a communications network, i.e., on a data path from a sender to a receiver, while limiting an increase in queuing delay along that path (see, e.g., Internet Engineering Task Force (IETF), RFC 6817). LEDBAT utilizes changes in the measured one-way delay along the data path to limit congestion which the flow itself induces in the network. LEDBAT is designed for use by background transfer applications, limiting interference with the network performance of competing flows. LEDBAT can be used as part of a transport protocol, such as TCP, or as part of an application, as long as the data transmission mechanisms are capable of carrying timestamps and acknowledging data frequently.
LEDBAT employs one-way delay measurements, based on a timestamp from the sender which each data packet carries, in order to detect an increase in queuing delay, which is an early signal of congestion. When the estimated queuing delay exceeds a predetermined target, or threshold, which is on the order of few tens to several hundreds of milliseconds, LEDBAT decreases the sending rate, using a congestion window at the sender side, in order to mitigate or prevent congestion in the network. The response of the sender, i.e., the decrease in sending rate by increasing the time interval between subsequent transmissions of data packets, is typically proportional to the difference between the estimated queuing delay and the target delay. The idle time period between subsequent transmissions is of the order of a few seconds or a few tens of seconds.
In the event that a communications network, such as a Local Area Network (LAN), Wide Area Network (WAN), Internet, or the like, comprises a Radio Access Network (RAN) through which the receiver access the communications network, the described congestion control mechanisms, and in particular LEDBAT, may have a negative impact on resource usage in the RAN. This situation relates, e.g., to a User Equipment (UE), such as a mobile phone, smart phone, tablet, computer, media player, or the like, downloading data from a server (the sending node or sender) via the RAN using a wireless radio-frequency access technology such as Universal Mobile Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE), Wireless LAN (WLAN) or WiFi, or the like.
Radio networks typically employ a state machine in the RAN and the UE to support radio states with different transmission bitrates over the radio interface and corresponding resource consumption, such as associated control signaling, allocated frequency bands and time slots, and power consumption in the UE. In UMTS, these states are referred to as Radio Resource Control (RRC) states. Some of the RRC states, and the possible transitions between the states (indicated by arrows), are illustrated in FIG. 1.
In general, radio states which provide the UE with a higher bitrate require more resources, and vice versa. The states in the RRC Connected Mode are, in order of decreasing bitrate and resource consumption: CELL_DCH (Dedicated Channel), CELL_FACH (Forward Access Channel), CELL_PCH (Cell Paging Channel) and URA_PCH (URA Paging Channel). The power consumption in the CELL_FACH is roughly 50% of that in CELL_DCH, and the PCH state uses about 1-2% of the power of the CELL_DCH state. In LTE, the radio state having the highest bitrate and resource consumption is the Active sub-state of Connected Mode.
Due to the delay and resource cost associated with switching radio states, switching to a lower radio state is only effected if the idle period is sufficiently long so that the resources conserved by switching to a lower radio state are sufficiently greater than the resources expended to switch to the lower radio state and then back to the higher radio state. Typically, switching to a lower radio state is triggered by detecting an idle period of data traffic using inactivity timers T1 and T2, illustrated in FIG. 1. The inactivity timers are configured by an operator of the RAN and are typically of the order of 2 seconds and 10 seconds, respectively.
In connection with congestion mechanisms such as LEDBAT, an increase of the idle time period between subsequent transmissions of data packets at the sender, as described hereinbefore, may result in a less efficient utilization of air interface resources as well as the UE battery. That is, the RAN and the UE may continue to reside in a high radio state while the sender has interrupted transmission of data packets for an idle time period in order to reduce the sending rate. During that idle period, until an inactivity timer triggers switching to a lower radio state, resources of the RAN and the UE are wasted for supporting a high-bitrate radio state which is not used for transmitting data to the UE. Furthermore, since the amount of data to be transmitted is not reduced, the RAN and the UE have to reside in higher radio states for a longer period of time. This is illustrated in FIG. 2, which shows transmission of three data bursts, each data burst comprising one or more data packets, for different scenarios.
The first diagram 210 illustrates transmission of data bursts 211-213, with idle periods 214 and 215 in-between subsequent bursts, and an inactivity timer of duration 216. When the first burst 211 is transmitted over the air interface, the RAN and the UE switch from a low radio state to a high radio state. Since the duration 216 of the inactivity timer exceeds the length of the idle periods 214 and 215, the RAN and the UE reside in the high radio state until the inactivity timer 216 triggers switching to the low radio state after transmission of the third data packet 213. The resource utilization of the RAN and the UE is illustrated in the lower part of diagram 210, where allocated resources which are not used for data transmission, i.e., wasted resources, are marked black 217.
If the idle periods in-between subsequent transmissions are increased, e.g., in response to detecting congestion along the data path, the situation depicted in diagram 220 of FIG. 2 may arise. In diagram 220, the idle periods 224 and 225 are increased as compared to diagram 210. Since the idle period 224 is still shorter than the duration of inactivity timer 216, the RAN and the UE will reside in the high radio state. Idle period 225, however, exceeds the duration 216 of the inactivity timer, resulting in a switching of the RAN and the UE to the low radio state, only to switch to the high radio state for transmitting the third data burst 223. As can be seen in the lower part of diagram 220, the amount of no-utilized radio resources (marked black 227) has increased as compared to diagram 210. This results in a less efficient utilization of radio resources, such as scheduled time slots and frequency bands, and a corresponding decrease in UE battery lifetime. In particular if congestion prevails for a longer period of time, and depending on the configuration of the sender's congestion time window and the inactivity timers of the RAN, the RAN and the UE are repetitively forced to switch to a high radio state when the sender resumes transmission of data, only to switch again to a less resource consuming radio state during idle periods.